1. Field of the Invention
The present invention relates to a method and apparatus for size measurements of particles of carbon black.
2. Description of the Related Art
Carbon black is a material of great industrial importance. It is formed by the incomplete combustion of certain fuels containing carbon. One of its major uses is to strengthen rubber tires. It is carbon black that gives tires their black color. It is also used to make black ink and black pigments, and toners for copy machines and printers. Every application of carbon black requires control of its particle size distribution. For example, tires require larger particle sizes than pigments and inks. To control the particle size distribution requires first being able to measure it. Carbon black particles are particularly difficult to measure because they are very small. A typical carbon black sample contains particles in the range of 40 manometers (nm) to one micrometer (μm) in diameter.
The standard method of measuring the particle size distribution of carbon black is using a transmission electron microscope (TEM) with an automated image analyzer. The particles are too small to be measured using an optical microscope. The procedure for using the TEM is very difficult and time-consuming to perform, and easier and faster methods have been tried. Centrifugal sedimentation has been used, but the parameter that is measured, Stokes diameter, is not a good representation of particle size. A non-spherical particle can be measured to have a wide range of Stokes diameters. The orientation of the particle with respect to its direction of travel affects the measured Stokes diameter. Laser diffraction analyzers have also been used, but they lack both the sensitivity and accuracy needed for measuring carbon black particles. In fact, laser diffraction analyzers are very inaccurate when measuring any particles that are not spherical in shape. The inherent inaccuracy is so great that it is incorrect to call laser diffraction analyzers particle size measurement instruments. Their reproducibility is very good, however, and the same sample is likely to be measured with nearly the same incorrect results every time. Electrical sensing zone (ESZ) instruments have also been used, but, while they have very good accuracy, they usually do not have enough sensitivity for measuring carbon black particles.
The ESZ instrument was first described in U.S. Pat. No. 2,656,508, issued to Coulter on Oct. 20, 1953. Some features thereof are shown in the Drawing. A particle 22 is measured by passing it through an electrical current-carrying aperture 18 between insulating containers 10, 20 holding a conductive liquid 21. The motion of particles through the aperture 18 is caused by a pressure difference across a partition 24 between the insulating containers 10, 20 produced by a vacuum or pressure source 14. The presence of a particle 22 in the aperture 18 increases the electrical resistance of the aperture 18 by displacing liquid 21 of equal volume to the particle 22 volume. The change in resistance may be detected as an increase in voltage across the aperture 18, or a decrease in current through the aperture 18. The change in resistance is approximately proportional to the volume of the particle 22. It is this approximate proportionality between aperture 18 resistance and particle 22 volume that gives the ESZ method high accuracy compared to several other methods of particle 22 size measurement. It does not matter if the particles 22 are electrically conducting or not, because all particles 22 in an ESZ instrument behave as if they are electrical insulators.
The sensitivity of the ESZ instrument can be increased by making the effective volume of the aperture 18 smaller. The smallest apertures 18 available commercially for ESZ instruments are not small enough for measuring the finer grades of carbon black particles 22. Usually, an aperture is made by boring a small hole through a thin sapphire wafer. The effective volume of the aperture can be made smaller by reducing the diameter of the hole, and making the sapphire wafer thinner. Unfortunately, a wafer thinner than about 30 μm is easily broken during normal operation of the ESZ instrument, and great care must be used to avoid breakage. U.S. Pat. No. 3,815,024, issued Jun. 4, 1974 to Bean and De Blois, describes a method of making very small apertures that can measure very small particles, but these are even more delicate than the small sapphire aperture mentioned above. The material used is polycarbonate, which has much less flexure strength and resistance to breakage than sapphire. My co-pending application Ser. No. 11/329,829, filed Jan. 12, 2006, describes a way to make an aperture of low effective volume that is resistant to breakage.
The medium carrying particles 22 through the aperture 18 is an aqueous electrolyte 21. Any given volume of electrolyte has a finite number of ions. This finite number produces a shot noise phenomenon that causes the noise to increase as the electrical current through the aperture 18 increases. An article by De Blois and Bean that describes their invention in greater detail than their aforementioned patent, in Rev. Sci. Inst. Vol. 41 No. 7, p. 909, 1970, mentions the shot noise, but does not identify the cause as the finite number of ions. The shot noise, which is lowest for uni-univalent electrolytes 21, decreases as the ion concentration increases. It is therefore desirable to use as high an electrolyte 21 concentration as possible. Unfortunately, it is difficult to disperse very small particles in highly concentrated electrolytes. The concentration that is typically used in an ESZ instrument is in the vicinity of 1% by weight of sodium chloride (NaCl). This concentration is high enough to screen electrostatic forces at the surfaces of the particles 22, and electrostatic repulsion cannot be used to keep the particles 22 separated from each other. Thus, to keep the particles 22 dispersed, a surface-active agent (surfactant) 23 must be added to the electrolyte 21. A surface-active agent 23 will usually not work if the equivalent concentration is more than about 4% by weight of NaCl. This is because high concentration can cause some of the surface-active agent 23 to flocculate and precipitate. The portion of the surface-active agent 23 still in suspension forms large micelles, which are detectable as noise by the ESZ instrument. If it were possible to increase the concentration further, and still keep particles 22 dispersed, noise could be reduced further. The prior art methods do not permit higher ionic concentrations in the ESZ instrument if very small particles 22 are used.